1. Field of the Invention
The present invention relates to a negative voltage generating circuit. A personal handy phone system (PHS) requires a power amplifier that consumes only a small electric power. Therefore, the power amplifier is constructed of a GaAs metal schottky field effect transistor (GaAs MESFET), which requires to be supplied with negative voltage. The present invention is used for such a purpose, for example, and provides a negative generating circuit that can be built in a chip together with the power amplifier.
2. Description of Related Art
A power amplifier constructed of a GaAs MESFET is required to be applied with positive voltage at the drain thereof and with negative voltage at the gate thereof. Therefore, a chip with such a power amplifier built-in is used by being connected to both a positive power supply and a negative power supply.
In order to obtain negative voltage by supplying positive voltage, a charging pump type DC--DC converter is used. The charging pump type DC--DC converter has four switches, two capacitors and one crystal oscillator. Switching FETs are generally used as the switches, and the crystal oscillator is connected to the capacitor having a large capacity.
When the chip containing the power amplifier constructed of the GaAs MESFET is used, the chip is connected to a positive power supply and a negative power supply comprising the charging pump type DC--DC converter.
The oscillation frequency of the oscillator in the charging pump type DC--DC converter is as low as approximately 10 kHz (= 1/100 .mu.sec). Because of the low oscillation frequency, a ripple is caused in the output voltage of the DC--DC converter, and hence, it is impossible to obtain a stable negative voltage. In order to stabilize the negative voltage, an inverting capacitor and a smoothing capacitor having large capacities are required. Moreover, since the oscillation frequency is low, a switching operation takes a long time, and hence, it takes a disadvantageously long time from the start of the switching operation until the negative voltage is stabilized. As still another problem, since the capacitors having large capacities are required, the size of the negative power supply becomes large.